Langmuir 1996,11, 693-695
693
Surface Chemistry on Colloidal Metals. Reversible Adsorbate-InducedSurface Composition Changes in Colloidal Palladium-Copper Alloys John S. Bradley,**+Ernestine W. Hill,? Bruno Chaudret,$and Anne Duteil$ Exxon Research and Engineering Company, Rt 22E, Annandale, New Jersey 08801, and Laboratoire de Chimie de Coordination, C.N.R.S., 205 route de Narbonne, 31077 Toulouse Cedex, France Received September 19, 1994. In Final Form: October 24, 1994@ Carbon monoxide adsorbsreversiblyinto organic suspensions of a series of colloidal poly(vinylpyrro1idone) stabilized bimetallic palladium-copper alloys with particle sizes of ca. 45 A, binding to both copper and palladium sites. The relative intensities of the [CuCOl and [PdCOlinfrared absorptionsof the as-prepared bimetallic colloids in the presence of saturated solutions of CO reveal the preference of palladium for surface sites on the alloy particles, in contrast to the more commonly observed surface segregation of copper in the bulk alloy. The IR spectrum of adsorbed CO at room temperature is time dependent over several days, in a manner suggesting a rearrangement of the surface atomic layers of the particles with further enrichment in palladium.
Introduction The structural and catalytic chemistry of GplO/Gpll alloys has received much attention, and in particular that of PdCu alloys has been extensively in~estigated.'-~The effect of alloying between Pd and Cu on the adsorption properties of the alloys and on their catalytic properties is well-known and the debate over the relative importance of electronic and geometric effects in catalysis has drawn heavily on such investigations. Bimetallic surface composition and structure are factors of central importance in any investigation of alloy surface chemistry. As part of our continuing work on the surface chemistry of transition metal ~ o l l o i d s we ~ - ~recently ~ reported the preparation of a series of poly(vinylpyrro1idone) (PVP) stabilized colloidal palladium-copper alloys with mean particle sizes ofca. 40A.5J1 The colloid particles prepared in this manner were shown to be homogeneously bimetalic for each compositionby single particle electron microprobe analysis. We also reported the infrared spectrum of carbon monoxide adsorbed on colloidalP d d & and assigned the three observed vibrational absorption bands a t 2093,2046, and 1936 cm-l (Figure 1)to linear CO on Cu (2093 cm-'), linear CO on Pd (2046 cm-9, and bridged CO on palladium (1936 cm-'1. We noted that the relative intensities of bands due to [PdCOl and [CuCOl in spectra recorded immediately after addition of CO seemed a t variance with the overall composition of the colloidal alloy and that the t Exxon Corporate Research.
* C.N.R.S.,Toulouse. @
Abstract published in Advance ACS Abstracts, February 1,
1995. (1)Gustafson,B.L.;Wehner, P. S.Appl. Surf.Sci. 1991,52,261-70. (2)Leviness,S.;Nair, V.; Weiss, A.; Schay, Z.; Guczi, L. J.Mol. Catal. 1984,25,131-40. (3)Clarke, J. IC A. Chem. Rev. 1976,75,291-305. (4)Ponec. V. Catal. Rev.Sci. Em. 1975.11. 41-70. (5) Bradley, J. S.; Millar, J. M.; Ifill, E. W.; klein, C.; Chaudret, B.; Duteil,A. In 20th International Congress on Catalysis; Aka6mia Kiado, Budapest Budapest, 1992;pp 969-79. (6)Bradley, J. S.;Hill, E. W.; Behal, S.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1992,4,1234-9. (7) Bradley, J. S.;Millar, J. M.; Hill, E. W.; Behal, S. J.Catal. 1991, 129,530-9. (8)Bradley, J.S.; Millar, J. M.; Hill, E. W.; Behal, S.; Chaudret, B.; Duteil,A. Faraduy Discuss. Chem. SOC. 1991,92,255-68. (9)Bradley, J. S.;Millar, J. M.; Hill, E. W. J.Am. Chem. SOC. 1991, 113,4016-7. (10)Bradley, J. S.;Millar, J. M.; Hill, E. W.; Melchior, M. J . Chem. Soc., Chem. Commun. 1990,705-6. (11)Bradley, J. S.;Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993,5,254-6.
spectrum was time dependent. We report here the results of further investigation into the behavior of the colloidal PdCu alloy surface and present evidence for the enrichment of the surface in palladium in the as-prepared colloid, in contrast with the more common, if slight, preference of copper for surface sites in CuPd alloys,12and also for a subsequent facile and reversible adsorbate induced enrichment of the surface in palladium in the presence of
co.
Experimental Section Palladium-copper particles stabilized with poly(vinylpyrro1idone) were prepared in colloidal suspension from palladium acetate and cooper acetate hydrate in 2-ethoxyethanolsolutions as described previously.11 Metal particles were characterized by transmissionelectronmicroscopy (PhillipsCM 12)and single particle electron microprobe analysis (EMA,Phillips EM 420 ST STEM 200 kev), which showed,as before, that the particles were uniformly bimetallic. Carbon monoxide was adsorbed on the colloidal alloy by passing a solvent-saturated stream of CO through CHzClz solutions ofthe PVP stabilizedcolloids. Infrared analysis was performed on a Perkin-Elmer 787 grating spectrophotometer or a Mattson Galaxy 5000 FTIR in dichloromethane solutions typically containing 4-20 mg/mL metal using 0.1-0.5 mm sealed solution cells with CaFz windows. Studieswere performed on a series of sampleswhich were shown by TEM and EMA to be homogeneously bimetallic, of approximately equal particle size (ca. 45 A),and not aggregated.
Results and Discussion Carbon monoxide is known to bind strongly to palladium surfaces but only weakly to copper surfaces under ultrahigh vacuum (UHV) ~0nditions.l~ CO desorption from copper surfaces occurs below 200 K, and CO has been reported not to bind to the surface of colloidal copper14at room temperature. However, alloyingwith a GplO metal increases the binding strength of CO on copper,15J6and CO adsorbs onto both Pd and Cu sites on supported PdCu alloy particles.17J8 Temperature programmed desorption (12)Surface SegregationPhenomena; Dowben, P. A., Miller, A., Eds.; CRC Press: Boca Raton, FL, 1990. (13)Niewenhuys, B. E. Surf. Sci. 1981,105,505. (14)Duteil, A.; QuBau, R.; Chaudret, B. M.; Roucau, C.; Bradley, J. S.Chem. Mater. 1993,5,341-7. (15)Rochefort, A.; Abon, M.; Delichere, P.; Bertolini, J. C. Surf Sci. 1993,294,43-52. (16)Campbell, C. T.Annu. Reu. Phys. Chem. 1990,41,775-837. (17)Sokolova, N. P. Zh. Fiz. Khim. 1976,50,536-9. (18)Hendrickx, H. A. C. M.; Ponec, V. Surf. Sci. 1987,192,234-42.
0743-7463l95I2411-0693$09.0QlO 0 1995 American Chemical Society
Letters
694 Langmuir, Vol. 11, No. 3, 1995
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Figure 1. Infrared spectrum of CO adsorbed on (a) PVP stabilized Pd50Cu50(ca. 45 A) and (b) Pddh63 (ca. 40 A) in
CO-saturated CHzClz.
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studies on CO on Pd(ll1) and PdCu(ll1) surfaces have shown that the heat of desorption of CO is lowered on the alloy compared to palladium.19 We have observed by infrared spectroscopy that CO adsorbs readily onto PVP stabilized colloidal PdCu particles in dichloromethane a t 25 OC.ll CO adsorption a t the metal surface takes place from a saturated solution of the gas (ca. 10 mM) and we therefore feel justified in the assumption that all the surface sites which are available for binding CO are covered.20 In order to infer the surface composition of the colloidal alloy particles by a simple comparison of intensities, it must be assumed that the extinction coefficients ccUcoand 6PdCO for Y [ C ~ C Oand I Y[pdcO] are identical. This is not necessarily the case. However here we are concerned only with changes in surface composition, and so we need only make the more plausible assumption that whatever is the relationship between E C ~ C O and EpdCO it is not composition dependent and therefore remains the same over the course of the experiments described. The relative intensities of the [CuCOl and [PdCOl bands for varying alloy compositions are therefore a significant indicator of the surface composition. In the following discussion of the infrared spectra of adsorbed CO, we use the assignments made previously,ll i.e. linear CO on Cu near 2090 cm-l, linear CO on Pd near 2040 cm-l, and bridged CO on Pd below 1950 cm-'. The infrared absorption of CO on Cu(1)in CHzClz solutions of [CU(CH~CN)~]+[BF~]occurs a t 2106 cm-l and Cu(1) is most probably a n intermediate in the reduction of Cu(I1) by the colloidal palladium surface, but the bands we assign to [CuCO] are consistently below 2095 cm-l and are thus assigned to CO on Cu(0). In the IR spectra of adsorbed CO on colloidal PdCu containing less than 50%copper the intensity ofthe [CuCO] absorption near 2090 cm-l is very low (Figure la), but above 60% copper the [CuCOl band is a major component of the spectrum (Figure lb). The total intensity of the linear [CuCO] and [PdCO] bands compared to the bridging [PdCO] band in Pd37Cu63, when compared with the same relative intensities for a pure colloidal palladium sample,6 reflects the dilution of the palladium surface by copper atoms, reducing the number of adjacent palladium atoms available as binding sites for bridging CO. No bridging CO between Cu and Pd is assigned, on the basis of comparison with the literature for supported copper-GpVIII alloys.l* It has been previously shown that adsorbed GO and free CO are in equilibrium in colloidal suspensions of paIladiumg and PdCu alloy^.^ Consistent with this equilibrium, CO is removed rapidly and reversibly from both Pd and Cu sites on the PdCu alloy colloid particles by passing a stream of solvent-saturated nitrogen through (19) Noordermeer, A,; Kok, G. A,; Niewenhuys, B. E. Surf Sei. 1986, 172, 349-62. (20) Asaturated solution of CO in CH2C12 contains an effective partial pressure of 200 Torr CO. In a solution containing 10 mg/mL metal, this corresponds to a n excess of lo2 of CO over total metal atoms, and with 45 A particles (ca. 30% dispersion) there is the equivalent of over 100 monolayers of CO in solution.
1938
Figure 2. Infrared spectrumof CO adsorbed on PVP stabilized Pd&&3: (a)as prepared; (b) after standing under CO (1bar) for 3 days; (c) after removal of CO and evacuation at 100 "C; (d) after readdition of CO.
the solution, thus reducing the concentration of dissolved CO. Reduction in infrared intensity occurs first for the [CuCO] band, consistent with the weaker binding of CO on copper compared to palladium,lgand the ease ofremoval of CO even from the strongly binding palladium site is probably a consequence of competitive adsorption by the polymer and/or solvent molecules. Readdition of CO after complete removal results not only in the recovery of both sets of IR absorptions but in a n increase in relative intensity of the [CuCO]band over the [PdCOl bands; that is to say the surface copper concentration has apparently increased during removal and readdition of CO. This observation raised the possibility that CO suppresses the surface copper concentration and that the original spectrum was not in fact a t a steady state. In order to test this hypothesis, a suspension of a PVP stabilized 45 A P d 3 7 C ~ colloidal 63 alloy was exposed to CO a t saturation concentration in CHzClz for several days. The spectra obtained are shown in Figure 2. After 3 days the v[cUco] band a t 2089 cm-I in the as-prepared sample (Figure 2a) has been totally suppressed (Figure 2b), and the spectrum contains only bands a t 2040 and 1960 cm-l resembling that for CO on PVP stabilized colloidal palladium.6 Removal of all adsorbed CO from the colloidal metal at this stage by evacuation under reduced pressure (0.01 Torr, 100 "C, 1h) (Figure 2c) followed by redissolution to the original volume and readdition of CO results in the recovery of the v [ c U c o band ~ a t 2089 cm-l in a spectrum recorded immediately after CO addition (Figure 2d). It is important for the following discussion that we regard the IR spectrum of the colloid under CO, when recorded immediately after CO addition, as reflecting closely the composition of the surface before CO addition. This is certainly valid since, as described above, the perturbation caused by the adsorbed CO occurs over a period of tens of hours, and in the short time elapsing between GO addition and spectroscopy, little change is likely to have occurred. We interpret these observations as follows. In 45 A particles of PVP stabilized colloidal Pd,Cul-, (x < 0.51, the copper has a preference t o dissolve in the palladiumrich matrix, maximizing CuPd contacts. This is consistent with the thermodynamics of PdCu alloy formation which is strongly exothermic21(the heat of solution of copper in (21) Hultgren, R.; Desai, P. D.; Hawkins, D. T.; Gleiser, M.; Kelley, K. K. Selected Values of the Thermodynamic Properties ofBinary Alloys; American Society for Metals: Metals Park, OH, 1973.
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Langmuir, Vol. 11,No.3, 1995 695
palladium is ca. -0.4 eV) but at odds with the reported the particle, over a period of several days. On removal of segregation of copper on clean palladium-copper alloy the CO a t 100 "C the original equilibrium is reestablished surfaces (see below). At the lower copper concentrations (demonstrating the mobility of surface and near surface the great majority of the copper atoms in the subject atomic layers at this temperature) with copper and colloidal alloys are thus internal, and the surface is palladium being repartitioned between surface and inpredominantly made up of palladium atoms. The sharp terior sites. The CO induced surface segregation of a Gp increase in the intensity of the [CuCO] band in spectra 10 metal in 10-11 alloys was reported some years ago by recorded immediately after addition of CO to colloids with Sachtler and co-workers for PtAu and PdAg303 and greater than ca. 50%copper content suggests that near subsequently for CuNi alloys by Wandelt et al. ,32 both on this concentration the interior of the particle is too the basis of photoemission spectroscopy. palladium-poor an environment for maximum CuPd The surface segregation of palladium over copper in interactions. These interactions must then be maximized the as-prepared colloidal alloys thus reflects the difference by the addition of copper atoms to the hitherto palladiumin relative stability of the two metals in the complex rich surface. Although a segregation of copper of varying chemical environment found a t the particle surface in the degrees has been reported for PdCu alloy s u r f a ~ e s ~ ~ rcolloidal ~~ system. The preference of palladium for surface calculations of the relative surface energies of the metals25 sites, even in the absence of the strongly binding CO, may show only a small difference between copper and palalso be due to structural and compositional effects which ladium. The difference is smaller than, for example, that operate a t small particle sizes in a manner we cannot yet between silver and palladium,26and PdAg alloys show a define. It is known that surface segregation effects are greater surface segregation of ~ilvel.2'3~~ than PdCu alloys crystal-face specific,32that segregation effects can penshow of copper. That copper and palladium have very etrate many layers from the surface of a n alloy,33and so similar surface energies is suggested by a comparison of the effect on structure of proximity to the surface is two separate embedded atom calculations which in turn certainly complex. At these particle sizes the great predict a segregation of copper and of p a l l a d i ~ m . ~The 6 , ~ ~ majority ofthe atoms in the particle are within four atomic small surface segregations reported experimentally reflect layers of the surface, and given the complexity of the CuPd the small energy difference between copper and palladium phase diagram there is no obvious way to predict either at the alloy surface, and so any perturbation of the surface the most stable structure of even single crystals ofPdXCul, by adsorbates (synthesis byproducts, polymer, solvent) or the preferred surface structure. We note, however, could drive this equilibrium to one side or the other. In recent publications of Toshima et al. reporting the the as-prepared state of the PdCu colloids palladium is preparation a t higher temperature, from the mixed stabilized a t the surface as evidenced by the low intensity hydroxides, of 18-Acrystalline colloidal PQCul, of the [CuCO] band after very short CO exposure. In the with the fcc structures predicted from the phase diagram. presence of adsorbed CO, however, an additional term is The distribution of copper in colloidal CuPd alloys is added to the energy balance, namely the higher binding under continuing study by EXAFS, the results of which energy for CO on palladium compared to copper, and under will be reported separately. Preliminary analysis confirms a CO atmosphere the surface palladium atoms show an the conclusion we reach here that copper has a tendency even greater preference for surface sites where they can to occupy interior sites and palladium is enriched a t the adsorb CO. Any surface copper atoms present in the surface. copper-rich alloys are slowly displaced into the interior of Acknowledgment. The authors thank C. Klein, E n o n (22)Sampath Kumar, T. S.; Hegde, M. S. Appl. Su$. Sci. 1986,20, Corporate Research, for electron microprobe analysis. 290-306. (23)Peacock, D. C. Appl. Surf. Sci. 1986,27,58-70. (24)van Langeveld, A. D.; Hendrickx, H. A. C. M.; Niewenhuys, B. E. Thin Solid Films 1983,109,179-92. (25) Chelikowski, J. R. Surf. Sci. 1984,139,L197-203. (26)Foiles, S.M.; Baskes, M. I.; Daw, M. S. Phys. Rev. B 1986,33, 7983-91. (27)Kuijers, F. J.;Ponec, V. J . Catal. 1979,60,100-9. (28)Noordermeer, A.;Kok, G.A,;Nieuwenhuys, B. E. Su$. Sci. 1986, 165,375-92. (29)Montejano-Carrizales, J. M.; Iniguez, M. P.; Alonso, J. A. Phys. Rev. B 1994,49,16649-58.
L4940748T (30)Bouwman, R.;Sachtler, W. M. H. J . Catal. 1970,19,127-39. (31)Bouwman, R.;Lippits, G . J. M.; Sachtler, W. M. H. J . Catal. 1972,25,350-61. (32)Wandelt, K.; Brundle, C. R. Phys. Reu. Lett. 1981,46,1529-32. (33)Vahalia, U.;Dowben, P.; Miller, A. J. Vac. Sci. Technol. 1986, A4, 1675-9. (34)Toshima, N.; Wang, Y. Adu. Mater. 1994,6,245-7. (35)Toshima, N.; Wang, Y. Chem. Lett. 1993,1993,1611-14.
